Answer :
Let
x -----> amount extra per month
we have that
120+12(30+x)=900
solve for x
120+360+12x=900
12x=900-120-360
12x=420
x=35
therefore
The answer is
120+12(30+x)=900 , $35
Let
x -----> amount extra per month
we have that
120+12(30+x)=900
solve for x
120+360+12x=900
12x=900-120-360
12x=420
x=35
therefore
The answer is
120+12(30+x)=900 , $35